Generally, very small telephone switching systems have been referred to as key telephone systems. Typically, a key telephone system was provided by extensive telephone line and control lead wiring between the key telephone sets with each telephone line extending to a telephone switching office. Each of the key sets included a plurality of interlocking pushbuttons or keys each for connecting the telephone set to a particular telephone line among a plurality of telephone lines routed to each key telephone set. The switching function of line selection was mechanically provided and distributed among the key telephone sets. The primary advantages of these systems were economy with small size and robustness. However, these early systems were difficult to expand, required extensive cabling, and were not compatible with the emerging world of digital switching and the associated digital communications.
In the late 1980's and 1990's, a new generation of small switching systems were developed to be compatible with the existing world of digital telephony. One of the better-known and most successful of these systems is marketed under the name of Norstar.TM. by Northern Telecom.
Further information on the Norstar digital key telephone system may be obtained from U.S. Pat. No. 4,873,682 issued Oct. 10, 1989 to G. F. Irwin et al. and U.S. Pat. No. 5,027,345 issued Jun. 25, 1991 to T. J. Littlewood; both patents are assigned to the same assignee as the present application and are hereby incorporated herein by reference. Yet other information on the Norstar system may be obtained from the publication "Telesis" 1989, two, published by Bell-Northern Research, the research arm of Northern Telecom Limited.
The Norstar system interfaces to the public switched telephone network (PSTN) via lines and trunks in the usual manner of a telephone switch. The core of the system named KSU (Key Service Unit) in Norstar, on the other hand, is connected to proprietary digital key sets via time-compression multiplex (TCM) links each carrying a pair of B-channels B1 and B2 at 64 kb/s and a D-channel at 16 kb/s.
The KSU can also be connected to a personal computer (PC). The physical and logical connection between a PC and the Norstar system is provided by a proprietary PC interface card that occupies one standard PC bus slot. A PC thus equipped can be connected to a Norstar system via a standard TCM port on the KSU. Firmware in the PC interface card provides PC applications with access to both D-channel functional messaging and B-channel data. The PC communicates with the card via interrupts for the D-channel and direct memory access for large data transfer associated with the B-channel traffic.
The Norstar system also provided for the connection of a PC and digital key set to a TCM port of the switch. A coupling device was inserted in the TCM path between the KSU and a digital station set to provide an interface to the TCM loop, a D and B1-channel interface to the station set and a B2-channel interface to an RS-232 link to a PC. The device included a modem to interface the B-2 channel to the PC. The device thus allowed the use of the PC to make data calls on the B2-channel and the use of a key set to make voice calls on the B1-channel.
The desirability of connecting both a telephone station set and a PC to a common port of a switching system is well recognized and is exemplified by numerous patents in addition to the Norstar system mentioned above.
For example, U.S. Pat. No. 5,003,580 to D. M. Duong describes an adapter for interfacing a work station terminal to a key telephone system. The adapter comprises a pair of interface circuits and a pair of relays for variously switching them to the work station or the station set.
U.S. Pat. No. 4,689,814 also describes an external device for interfacing an external device such as a PC and an electronic telephone to a switching system. This device also uses a switching module comprising relays to switch the lines from the ports on the switch to the external devices. The switching module uses a resistor network to mute the audio signal to the telephone set when an external device is actuated.
The above and other similar patents therefore indicate that it is known to provide an adapter device for interfacing a telephone station set and a personal computer to a telephone line from a switching system. It is also known to provide a PC programmed to function cooperatively with a station set so that it may be used to initiate telephony functions. However, the known devices are not adapted to provide simultaneous telephony functionality and data connectivity such as to the Internet network, and an attached set.